CHICAGO — In the wake of the deadly explosions at the finish line of the Boston Marathon, officials in Chicago said they were stepping up announcements on trains and buses, but stressed there is no known threat in Chicago.

"We are increasing the frequency of announcements that say, 'If you see something, say something' on trains and buses to encourage riders to be alert," CTA spokeswoman Tammy Chase said.

Metra also is boosting precautions at Downtown stations and aboard trains.

"In response to recent events, you may see an increased police presence at Chicago Downtown terminals and onboard trains," the commuter rail agency said.

Mayor Rahm Emanuel called his counterpart in Boston, Mayor Tom Menino, to offer Chicago's support. And Chicago's Office of Emergency Management and Communications was in touch with law enforcement around the country.

"We are closely monitoring events as they unfold in Boston, and the City of Chicago’s public safety departments are actively communicating with our law enforcement partners both locally and throughout the country," city spokeswoman Melissa Stratton said. "At this time, we are not aware of any additional threats."

Hundreds of Chicagoans were among the thousands participating in the Boston Marathon, and friends and family members were anxiously awaiting word from them Monday in the moments after two bombs at the finish line rocked the world-famous race.

Three people are reported dead, including an 8-year-old boy, and more than 100 injured, according to theBoston Globe. Many were taken to local hospitals with severe injuries, the Boston Globe reported. Officials said two bombs detonated near the finish of the race on Boylston Street in downtown Boston just before 2 p.m. Central Daylight Time.

The bombs were 50-100 yards apart, according to Boston Police Commissioner Ed Davis, who said there was no warning of the blasts.

"There were two bombs that exploded near the finish line in today's Boston Marathon," marathon officials posted on the race's Facebook page. "We are working with law enforcement to understand what exactly has happened."

She had finished the race and already returned to her hotel to pack up for the flight back to Chicago when she heard the news.

"I was getting all these emails and texts," Jaehn said from Boston Logan Airport. "'Congrats' eventually turned to 'Are you OK? Are you OK?' "

The marathon's list of entrants included 361 runners from Chicago.

CARA sponsored two busloads of runners — a total of 110, said Matt DenBesten, CARA's organization manager.

Jaehn said everyone she got in touch with said they were safe and working to reach out to others.

A group of 86 runners associated with Fleet Feet Sports Chicago went to Boston to participate in the event. All of the participants were accounted for, said Mark Colpoys, the retailer's vice president.

"We are trying to assist people tonight who have missed their flights," Colpoys said. "In some cases people are buddying up in hotel rooms. We're just making sure people have a place to stay."

Mayor Emanuel, in a statement, said he called Menino, "to convey support from the people of Chicago. During this time of tragedy and uncertainty, the people of Boston are in our thoughts and prayers."

"The running of the Boston Marathon and Patriots Day are time-honored traditions," Emanuel said. "While the details of today’s tragedy are still unclear, one thing was immediately known: the patriotism and professionalism of public servants and first responders. Our hearts go out the first responders, runners, volunteers and spectators in Boston today."

One of the runners likened the scene to the aftermath of the Sept. 11, 2001, terror attacks.

"It was like 9/11," said Michael Arnstein, 36, of New York City, who completed the race with his wife five minutes before the blasts. "When that second bomb went off, everybody panicked."

Arnstein told DNAinfo.com New York that he was 900 feet from the explosions and described how "The first one went off, and everything shook."

"There was a big cloud of smoke, and then a few seconds later, a second thing went off, and that was confirmation," he said.

Runner Matt Villicana, 28, who works as an environmental scientist at Tetra Tech Inc. in the Loop, had already finished the marathon and was back at his hotel several blocks away when the explosions went off.

"I heard the first explosion, and the room shook. The second one, I just heard," he said. With cellphone service down, Villicana was able to get ahold of family and friends online.

"It was kind of nerve-wracking," said Villicana's father, Mel Villicana, about not being able to reach his son immediately by phone. "I was a little panicky."

Matt Villicana was told about 4 p.m. Monday that his hotel, along with others in the area, would be on lockdown for 24 hours.

"It's just been really confusing," he said. "Police are saying to stay in the hotel. It's kind of frightening because one of the bombs was found in a hotel."

Chicago runner Jill Elaine Czarnik, 24, had crossed the finish line just 20 minutes before the explosions went off.

Standing in a hospitality area about a quarter-mile from the finish line, she called it "kind of like a movie scene."

Czarnik told ABC News: "It's just very weird ... because you don't know what it is, and you're just very delusional because you just ran a marathon."

Runner and blogger Pete B. of Chicago entitled his post Monday at lakefronttrail.blogspot.com "Chaos at Boston."

"Explosions went off near the Boston Marathon finish line just about 70 minutes after I finished the race. The euphoria of my finish has evaporated, and I feel sick for all of the victims, who were mostly spectators.

"I am staying near one of the hospitals. There are a lots of ambulances going under my hotel window. The people inside were cheering me on this afternoon."

Runners were treated in the tents beside the race route, and others were whisked away by ambulance.

The Chicago Marathon is scheduled for Oct. 13. Some runners, in the moments after Boston's horror, said they were nervous about participating.

"Not going to lie that I'm strongly considering not running in the Chicago Marathon at this point...," tweeted @mfrost503.

"I'm still not deterred from running the Chicago Marathon this year. My spirit is still high. I'll be at the start and finish on Oct. 13th," tweeted @DaniellleHarth.

In a statement, Chicago Marathon officials expressed sympathy for victims of the Boston attacks.

"The Bank of America Chicago Marathon is deeply saddened by the situation in Boston today, and our thoughts and prayers are with our colleagues at the Boston Marathon, and all of the spectators, participants, their families and friends."

At chicagorunnersblog.com, people posted with a mix of horror and anger.

"As a runner, I feel like someone just attacked my friends. As a person, I'm just horrified," said one.

"Running is a sport of love and support. Who would do such a thing?" said another.

In the wake of the blasts, security was ramped up in Washington, D.C., and New York City.

Anyone with information is asked to call Boston investigators at 800-494-TIPS.